California is experiencing more mudslides, which leads me to wonder: why hasn’t everything that’s prone to slide in wet conditions slid already? How can there always be still more precariously balanced hills waiting to break loose?
Most of them have slid before and they’ll do it again. Once one layer lets go and slides down a mountain, guess what’s underneath? More slidey stuff just waiting for enough rain.
Erosion, removal of vegetation through cutting or fire, and other factors can destabilize slopes that were fairly stable before.
And of course this newly exposed level doesn’t have vegetation to secure it when it becomes saturated so it is prone to sliding, as are slopes where vegetation has been destoryed by fire or removal.
It should also be notes that the composition of the south facing San Gabriels and San Bernardino moutains are a combination of loosely aggregated metasedimentary rocks and quartz sandstone which are very incompletely metamorphized into solid rock and thus are hoghly subject to stress-induced fracture from snow, ice, or absorbed water loads. The “mud” that flows from these mountains is really a slurry of water, mud, organic material, and rocks of various sizes from sand to boulders. The mountains are not running out of any of this.
Stranger
Take a look at some pictures of alluvial fans. All the stuff up on top of the mountains ends up coming down eventually.
That is a good point as well; material gets funneled down the fan, which also often happens to be the location where people like to build because it is a watershed that support foliage.
Anyone really interested in this topic should read John McPhee’s The Control of Nature. He devotes a third of the book to the attempt to control mudflows and rock slides in the San Gabriel Mountains above Pasadena/Altadena/La Canada (northern suburbs of Los Angeles) and how even the extensive efforts still fail to fully control damage during the seasonal heavy rains.
Stranger
Those other factors might be grading and leveling for roads and housing in previously undeveloped areas. This changes water courses and seepage, sometimes in unexpected ways.
Given enough time, the mountains will indeed erode away into nothing. However, since California’s a seismically active zone, the mountains continue to rise in elevation due to uplift from periodic earthquakes. It’s a process that’s been ongoing for hundreds of millions of years.
Nitpick: billions. It’s as old as the plane, basically.